The present invention relates to a coated cutting tool (cemented carbide insert) particularly useful for wet and dry milling of low and medium alloyed steels, with raw surfaces such as cast skin, forged skin, hot or cold rolled skin or pre-machined surfaces.
When milling low and medium alloyed steels with cemented carbide tools, the cutting edge is worn according to different wear mechanisms, such as chemical wear, abrasive wear, adhesive wear and by edge chipping caused by cracks formed along the cutting edge, the comb cracks.
The comb crack formation is particularly severe when wet milling is employed (using coolant). Coolant and work piece material may penetrate and widen the comb cracks and the edge will start to chip. A chipped edge will generate a bad surface finish of the machined component.
Different cutting conditions require different properties of the cutting insert. For example, when cutting in steels with raw surface zones, a coated cemented carbide insert must consist of a tough carbide and have very good coating adhesion. When milling in low alloyed steels the adhesive wear is generally the dominating wear type. Here preferably thin (1-3 μm) CVD- or PVD-coatings have to be used.
Measures can be taken to improve the cutting performance with respect to a specific wear type. However, very often such action will have a negative effect on other wear properties.
The influence of some possible measures is given below:                1.) Comb crack formation can be reduced by lowering the binder phase content. However, such action will lower the toughness properties of the cutting inserts which is not desirable.        2.) Improved abrasive wear can be obtained by increasing the coating thickness. However, thick coatings increase the risk for flaking and will lower the resistance to adhesive wear.        3.) Milling at high cutting speeds and at high cutting edge temperatures requires a cemented carbide with a rather high amount of cubic carbides (solid solution of WC-TiC-TaC-NbC). Such carbides will more easily develop comb cracks.        
So far it has been very difficult to improve all tool properties simultaneously. Commercial cemented carbide grades have therefore been optimized with respect to one or few of these wear types and hence to specific application areas.
Swedish patent application 9501286-0 which corresponds to U.S. Ser. No. 08/616,012, herein incorporated by reference discloses a coated cutting insert particularly useful for dry milling of grey cast iron. The insert is characterized by a straight WC-Co cemented carbide grade and a coating including a layer of TiCxNyOz with columnar grains and a top layer of fine grained textured α-Al2O3.
Swedish patent application 9502640-7 which corresponds to U.S. Ser. No. 08/675,034, herein incorporated by reference, discloses a coated turning insert particularly useful for intermittent turning in low alloyed steel. The insert is characterized by a WC-Co cemented carbide body having a highly W-alloyed Co-binder phase and a coating including a layer of TiCxNyOz with columnar grains and a top layer of a fine grained, textured α-Al2O3.
Swedish patent application 9503056-5 which corresponds to U.S. Ser. No. 08/703,965, herein incorporated by reference, discloses a coated turning cutting tool particularly useful for cutting in hot and cold forged low alloyed steel. The insert is characterized by a WC-Co cemented carbide body having a highly W-alloyed Co-binder phase and a coating including a layer of TiCxNyOz with columnar grains and a top layer of a fine grained, α-Al2O3.